Noise neutralizing demodulator



Patented Dec. 9, 1952 1i- NOISE NEUTRALIZING DEMoDULATon Donald L. Hings, Vancouver, British Columbia,

' Canada Application April 6, 1948, Serial No. 19,24'1'v 11 claims. (c1. 25o-20) My invention relates in general to radio circuits for the elimination of interference noise and in particular to converter or detector Systems having noise eliminator circuits.

An object of my invention is to provide an improved carrier wave detection system and more specically to the demodulation of only the intelligence on a carrier wave having interference Wave energy super-imposed.

Another object of my invention is lto prevent portions of the carrier wave energy being rendered undetectable from impulse wave shock.

. Another object of my invention is to provide increased selectivity in a carrier wave amplifier without materially increasing the length of the oscillatory ltrain caused by high amplitude shock.

A still further object of my invention is to obtain an audio-voltage free from interference wave energy by obtaining energy from the sidebands outside the intelligence sidebands and inverting and applying this energy to cancel or neutralize the interference wave energy on the carrier.

Other objects and a fuller understand of my invention'may be had by referring to the following description and claims, taken in conjunction with the accompanying drawing, in which:

Figure 1 is a schematic diagram of the preferred embodiment -of my invention;

Figure 2 is a graph of the carrier Wave input voltage with high interference peaks superimposed thereon;

Figure 3 is a graph oi the second carrier voltage used as a control of the rst carrier wave;

Figure 4 is a graph of the voltages theoretically obtainable across the rst carrier lter input terminals to describe one portion of the operation; v

Figure 5 is a graph of a neutralizing voltage theoretically'obtainable across the rst carrier vinput terminals used to describe another portion of the operation of my circuit;

Figure 6 is a graph of the actual voltage obtainable across the input terminals of lthe rs carrier iilter;

Figure 7 is a graph of the voltages obtainable on the detector load; and

Figure 8 is a graph of the audio-output voltage of my system.

This application is an improvement on my apvpiicauon Serial r172,509, med september 6, 1947,

wave having superimposed thereon high peaks of interference. Such a source may be, as shown in Figure 1, an I. F. transformer I I for applying such rst carrier wave to an amplifier 'stage 40. This amplifier stage 69 includes in genera] a thermionic tube shown as an I. F. amplifier tube I2. This tube I2 has a cathode I6, control, screen and suppressor grids I5, III and I3 respectively, and an anode Il. An improved feature of my invention is the provision of some form of a limiterto prevent high positive peaks of inter# ference from saturating the tube I2. In my invention I show a diode circuit I3 for shorting or by-passing to ground these peaks of interference exceeding a given value. y

A terminal I9 is provided at the lower end of the secondary of the I. F. transformer II'for applying thereat a bias potential and providing a grid-cathode return path which grid-cathode return path is, for the best operation of my circuit, of low D. C. impedance.

The amplification of this amplifier stage 40 is preferably controlled by some form of A. V. C. action. I show a first A. V. C. network 35 deriving a voltage from the anode Il for obtaining at the terminal 31 an A. V. C. voltage adapted to control the gain of the ampliiier tube I2. An A. V. C. circuit 35 is used to obtain the correct polarity `and degree of voltage required for this automatic gain control which may be applied to the screen grid I4 or, by obvious modication, may 'be applied to the amplifier tube I2 in other well-known manners, which should not materially increase the grid-cathode return path imped ance.

|The output of this amplier stage 40 is applied to a coupling means which' may bein the form of an I. F. transformer with the Secondary thereof comprising a first carrier input v20. A low pass lter 2I having two input terminals 4I and 42 is adapted to be connected in series with this first carrier input 20. In this preferred embodiment, I show this second inputterminal 42 as being connected to ground. I next provide some form of amplitude discriminating means connected across this series combination; and I prefer to show such amplitude discriminating rst circuit means 43 in the form of a periodic or varying voltage which I term a source of second carrierwaves and a `unilateral current passing device, shown as a rst rectifier 22.` I show a generator 25` for producing this second carrier 'wave as being connected in 4series with this rst rectier22 -to comprise this first circuit means'li'3I which is connected 3 in parallel with the series combination of the first carrier input and the input terminals 4| and 42 of the low pass lter 2|. Also provided in my system is a second circuit means 44 which performs the function of frequency discrimination and, in this preferred embodiment, shown as being a unidirectional path for passing currentl in the, opposite direction from said rst circuit means relative to said first. carrier input 20. This second circuit means 44 in this improvement is shown as including two parallel paths, one of which includes a second rectier 24 and a first parallel resonant circuit 23, and the other including a third rectifier 26 and a second parallel resonant circuit 21'. These second and third rectifiers 24 and 2G accomplish the unidirectional passage of current in said second circuit means 44. The lower ends. of the first and second parallel resonant circuits 23 and 21 are connected to ground to complete the circuit. The low pass filter 2|, which may be designated as a first carrier wave filter for removing this first carrier Wave, has an output terminal 45 for producing an output voltage relativegtc ground or the terminal 42. The output of this low pass lter 2| is applied to a load 29 and a detector 28;. This detector 28` must be connected in the same sense as the second and third rectifiers 24 and 26 with respect to ground. Thus the detector 28 and the load 29 are effectively connected in series across the output terminals 45 and 42 of the 10W pass lter 2|'. The energy from the load 29 is next applied through an isolating section or'second load including a paraliel combination of a resistor 3| and a condenser 32v to ground. The isolating rectifier 30 and its load 3|32 must be connected in the same sense relative to ground as the detector 28and itsload 29..

The energy across the second loadY 3'I-32 is thereupon applied to a second carrier filter 33. This second carrier filter. 33 is designed toremove the second carrier wave obtained from the generator and thereupon deliver to an audiooutput terminal 34 anaudio-voltage or other voltage corresponding to the intelligence on the carrier wave whichV is substantially free from any interference.

The A. V. C. voltage obtainedat the terminal' 31 may be used to. control through. a lead 3B, the amplitude of the, second carrier waveV generated .by the generator 25. By thus providing A. V. C. control on both the generator 25 and the I. F. input or rst carrier input 2|), a fairly constant ratio of iirst carrier wave to second carrier wave amplitudeis maintained.

Operation In the operation of' my circuit, the diode circuit I8 is used to limit any spurious or high positiveA peaks of interference which would normally saturate the amplifier tube. I2. Such a rst carrier voltage with interference peaks superimposed thereon. may be such as is shown in Figure 2; These high positive peaks are bypassed to ground through the diode circuit |8 and thus are not applied to the control grid I4 which would otherwise saturateA the tube I2. The rst carrier wave that isthus applied to theV amplier stage 4l1`has these positive peaks of interference limited to a value which the tube I2 can amplify without distortion. This amplified aud limited first carrier wave is thus applied at the rst carrier input 20;

The first circuit means 43 which has been described as an amplitude discriminating means uses the voltage of the second carrier Wave 49 to accomplish a gating function. The second carrier Wave 49 may be as shown in Figure 3. This second carrier wave is arranged to be maintained at an amplitude greater than the maximum amplitude of the modulation or intelligence on thev first carrier input 23. Such a ratio of first carrier wave amplitude to second carrier Wave amplitude may be accomplished by the A. V. C. network 35 and the A. V. C. circuit 33. This A. V. C. voltage may be used to control either one or both of the amplifier stage 40 and the. second carrier wave generator 25. By virtue of the second carrier wave having an amplitude greater than the first carrier wave, this gating function is accomplished during half cycles of the second carrier wave. wave thus permits a voltage to appear across the input terminals 4| and 42 of the rst carrier filter 2| during half cycles of the second carrier Wave which will be during positive half cycles of the second carrier wave for polarity of the rectiers as shown in Figure l, since the input terminal of the low pass filter 2| presents a capacitive load.

The following theory is offered to help in the understanding of the operation of my circuit.' This operation can be shown by considering theL first carrier wave input 28, such as shown in* FigurevZ as being composed of a modulated first. carrier wave 46 with a modulation component 41 and having superimposed thereon a high unimpeded; whereas, conduction through the first circuit means 43 would only result at the instant that the first carrier voltage would be of a higher amplitude than the second carrier During such time, the net voltage rei maining on the input of the low pass filter 2| A would be gated by the second carrier and limited voltage.

to a maximum amplitude of the amplitude of the second carrier, as shown-in Figure 4.

The dotted lines'5 in Figure 4 represent the maximum instantaneous voltage which is. permitted to appear on the rst carrier filter input Assume that the short terminals 4I and 42. circuit is to be removed from the tuned circuits 23 and 21, and these two circuits tuned respectively; to each side of the rst carrier frequency and suiiicientlydisplaced so as to present low impedance to the intelligence sidebands; the effect of these tuned circuits will thus be negligible so far as the intelligence modulation is concerned. However, insofar as components of the interference are concerned which are outside ofl the intelligence sidebands, the tuned circuits 23 and 21 present avery high impedance. The eiect. is to prevent conduction of these interference frequencies through the second circuit means 44; however, conduction of these frequencies is permitted through the first circuit means 43, restricted only by the amplitude discrimination accomplished bythe second carrier voltage. The result of the current flow through this first circuit means 43 is to impose a net neutralizing voltage 5| on the input terminals ofV the first carrier filter 2| which is of opposite polarityV to that shown in Figure 4. This neutralizing Voltage 5| will be such asy shown in Figure 5. The voltage shown in Figures 4 and The second carrier 'do `not appear individually on the input terminals of the first carrier lter 2l but rather combine instantaneously to result in a voltage such as that shown in Figure 6. It can now be seen that the part of the high interference peaks 48 which remained in Figure 4 as a limited interference peak 52 has been neutralized by this neutralizing voltage 5I resulting in a portion 53 of the modulation component 41 being somewhat deformed but substantially free from interference. Thus the action of the amplitude discriminating and frequency discriminating circuit means 43 and 44 may be considered as preventing portions 53 of the carrier wave energy from being rendered undetectable from impulse wave shock of the interference peaks 48.

It is obvious that with the detector 28 being connected in the proper polarity, the resultant voltage on the load 29 will be such as shown in Figure 7, the irst carrier component having been removed by the iirst carrier filter 2|. This load output voltage 54, as shown in Figure 7, can be seen to have an amplitude corresponding to the modulation component 4l which appeared on the first carrier input 2B substantially free from the high interference peaks 43 but showing the gating action of the second carrier Wave 49. It remains to remove the second carrier component in this voltage wave 54 by means of the second carrier filter 33 presenting on the output terminals 34 and ground an audio-output voltage 55 shown in Figure 8, which is a voltage corresponding to the modulation component 4"! or the original intelligence on the iirst carrier input 2U.

Another improvement on my invention is the provision of the isolating rectier 30. The action of the isolating rectiiier is to prevent interaction of the second carrier lter 33 on the rst carrier filter 2|.

In addition to the noise neutralizing properties, the system evidences a capacity for increasing the effective selectivity of the amplifier stage to which output it is connected, over and above the selectivity obtained by a combination of the same amplifier stage and a normal detector output. It is evident that this action results from the fact that those frequencies to which the sideband circuits present a high impedence cannot appear in the output. Thus the amplifier stage has an increased selectivity without increasing the length of the oscillatory wave train caused by high amplitude shock from interference noise energy.

Although my invention has been described in its preferred form with a certain degree of particularity, it is understood that the present disclosure of the preferred form has been made only by way of example and that numerous changes in the details of construction and the combination and arrangement of parts may be resorted to without departing from the spirit and the scope of my invention as hereinafter claimed.

What is claimed is:

1. A radio circuit comprising, a source of intelligence modulated first carrier Waves having superimposed thereon high peaks of interference, a first c-arrier wave lter having two input terminals, means for serially connecting said source with said two input terminals and two parallel paths of opposite sense, a source of second carrier w-aves having a maximum amplitude kgreater than the maximum amplitude of the intelligence component of said rst carrier wave and less than the amplitude of said interference peaks, means for connecting into one of said -paths said second carrier wave source for gating the conduction through said one path for thereby producing across said input terminals at said iirst carrier frequency a unipolarity voltage having at any instant an amplitude less than the amplitude of said second carrier YWave and thus limiting said interference peaks to that amplitude, at least one parallel resonant circuit connected into the other of said paths and presenting a low impedance to the intelligence component of said rst carrier wave thereby producing a negligible affect on said Vlimiting action and presenting a high impedance to frequencies other than those of the intelligence sidebands for thereby producing across saidinput terminals a voltage of opposite polarity to said unipolarity voltage resulting in the reduction of the limited interference peaks, said iii-st carrier Wave lter having two output terminals, a load and a rectier serially connected across said two output terminals with the rectiier connected in a sense so as to pass voltages of the polarity of said unipolarity voltage, and a second carrier filter connected across said load for removing said second carrier wave thereby lproducing across the output terminal-s of said second carrier wave lter said intelligence substantially free of said interference.

2. A radio circuit comprising a source of intelligence modulated rst carrier Waves having superimposed thereon high peaks of interference, a load and a detector serially connected with said source, circuit means passing current in only one direction, connection means for connecting said rst circuit means in parallel with said series combination, second circuit means connected in parallel with said iirst circuit means for passing current in the opposite direction relative to said sour-ce, a volta-ge source serially connected in said rst circuit means and having a maximum amplitude greater than the maximum amplitude of the intelligence component of said first carrier wave and less than the amplitude of said interference peaks, said detector and said first circuit means passing current in the saine direction relative to said carrier wave source, said second circuit means including two parallel paths, one of which includes a rst rectier and a first parallel resonant circuit and the other including a second rectier and a second parallel resonant circuit, said first and second rectiiiers accomplishing said passage of current in said opposite direction, said first and sec-ond parallel resonant circuits having a resonant frequency higher and lower respectively than the frequency of said rst carrier wave thereby presenting a low impedance to the intelligence component of said first carrier Wave and presenting a high impedance to frequencies other than those of the intelligence sidebands for thereby producing across said load a voltage corresponding to said intelligence substantially free from said inte-rference.

3. A circuit comprising a source of intelligence modulated carrier waves capable of having superimposed thereon interference energy, first and second parallel unidirectional paths of opposite sense, a load and a detector serially connected with said parallel paths, and means for injecting energy from said source into said series combination, said first path including dis'- criminating means which are substantially only amplitude discriminating and said second path bined voltages of a frequency between that of said rst and second carrier waves, means for passing half cycles of said selected component of said second polarity, and means for selecting an average voltage of a frequency less than that of said second carrier Wave.

DONALD L. HINGS.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,029,354 Bossart Feb. 4, 1936 2,087,063 McCutchen July 13, 1937 Number 

